1. FIELD OF THE INVENTION
The present invention relates to an image forming apparatus having a plurality of developing devices.
2. DESCRIPTION OF THE RELATED ART
Copying machines having a plurality of developing devices containing toners of different colors have become common in recent years. Toner replenishment in the aforesaid copying machines is accomplished by two kinds of toner replenishment, to wit, routine toner replenishment and non-routine emergency toner replenishment. Routine toner replenishment, for example, indicates toner replacement wherein a specified quantity of toner is replenished at a specified timing to replace an estimated quantity of consumed toner used in the normal operation of the copying machine, to wit, so as to replace a specified quantity of toner consumed by the developing operation or replace a specified quantity of toner consumed by toner dust dispersion and the like which is unrelated to the developing operation.
In contrast, non-routine emergency toner replenishment indicates toner replenishment to prevent a reduction of toner density when a condition arises wherein toner consumption is greater than that during normal copying machine operation. The aforesaid emergency toner replenishment is accomplished by having the operator actuate an input means for inputting a toner replenishment instruction.
For example, when an original document having a dark image density is copied repeatedly, the quantity of consumed toner is greater than the quantity of toner estimated for normal consumption, such that the previously described routine toner replenishment is not equivalent to current toner consumption. Thus, a non-routine toner replenishment is accomplished by the previously mentioned input means so as to avoid a reduction of copy image quality.
The aforesaid non-routine toner replenishment in the previously described copying machine is conventionally executed only for the selected developing device.
For example, when a developing device accommodating black toner is selected and the previously mentioned input means is actuated by the copying machine operator, non-routine toner replenishment is executed only for the developing device accommodating black toner.
Accordingly, when non-routine toner replenishment is desired for a developing device which has not been selected, said developing device must be selected by a specified operation and thereafter the aforesaid input means must be operated.
In the emergency toner replenishment operation for individual developing devices, the waiting time until emergency toner replenishment is directed for the next developing device is unreasonable because mixing of the developing material and the like requires a long time.
That is, in a copying machine having a plurality of developing devices, a long time is required to accomplish non-routine toner replenishment and the number of steps in the operation is quite complex.
Further, when the aforesaid copying machine is capable of producing color images by mixing three primary colors of red, green and blue, and non-routine toner replenishment is executed only for the developing device selected by the operation of the previously mentioned input means, differences accrue in the quantities of toner accommodated in each of the various developing devices leading to unsatisfactory color matching in the copy images.